1. Field of the Invention
This invention relates to a method of placing an electronic responder in an electrically conductive article, the responder being of the type having at least one coil designed as an antenna, which, in operation, receives an alternating magnetic field produced by an interrogation means.
2. Description of the Related Art
One example of such a responder is described in U.S. Pat. No. 4,196,418. This known responder comprises a coil forming part of a resonant circuit tuned to the predetermined frequency of an interrogation field. The responder further includes a code generator controlling a switch means which, in operation, varies the electrical properties of the resonant circuit in the rhythm of a code stored in the code generator. This variation can be detected by means of the interrogation means.
Although the responders described can be used in a great many situations for detection and identification purposes, the use of such responders for detection or identification of electrically conductive objects, such as metal articles or at any rate objects containing a large quantity of electrically conductive material has not been possible up to now.
The fact is that a good operation of these known and similar responders requires a good magnetically inductive coupling to be established between the at least one antenna coil of the responder and the antenna coil(s) of the interrogation means through which the interrogation field is produced. This means that the field lines of the interrogation field should at least partly extend through the region enclosed by the at least one antenna coil.
This condition can be met in a simple manner if both the coil of the interrogation means and the antenna coil of the responder are present in air or a different medium which is not an electrical conductor. However, if the responder is present on a metal article or is even recessed therein, no magnetic coupling is possible.